1. Field of the Invention
The invention relates to microwave vacuum tube devices.
2. Discussion of the Related Art
Microwave vacuum tube devices, such as power amplifiers, are essential components of many modern microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they generally lack the power capabilities required by most microwave systems. Microwave vacuum tube amplifiers, in contrast, can provide higher microwave power by orders of magnitude. The higher power levels of vacuum tube devices are the result of the fact that electrons can travel at a much higher velocity in a vacuum with much less energy losses than in a solid semiconductor material. The higher speed of electrons permits a use of the larger structure with the same transit time. A larger structure, in turn, permits a greater power output, often required for efficient operations.
Microwave tube devices typically operate by introducing a beam of electrons into a region where the beam interacts with an input signal, and deriving an output signal from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr., Microwave Tubes, Artech House, 1986, 191–313. Microwave tube devices include gridded tubes (e.g., triodes, tetrodes, pentodes, and klystrodes), klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All these devices contain the basic components of a cathode structure, an interaction structure, and an output structure. (A grid is generally used in the cathode structure, to initiate emission from electron emitters, and the grid can also be used to modulate the electron emission to get a desired output. As used herein, grid indicates any structure that controls electron emission from the cathode, and the grid can have, for example, multiple apertures or a single aperture.)
These devices are typically formed by mechanical assembly of the individual components, e.g., aligning and securing the individual elements on a supporting structure. Unfortunately, such assembly is not efficient and cost-effective, and inevitably introduces some misalignment and asymmetry into the structure. Some attempts to address these problems have led to use of sacrificial layers in a rigid structure, i.e., a structure is rigidly built with layers or regions that are removed in order to expose or free the components of the device. See, e.g., U.S. Pat. No. 5,637,539 and I. Brodie and C. Spindt, “Vacuum microelectronics,” Advances in Electronics and Electron Physics, Vol. 83 (1992). These rigid structures generally reflected an improvement, but still encountered formidable fabrication problems, such as alignment issues and parasitic effects. Thus, improved fabrication methods are desired.
Improvements in the emission source of such microwave tube devices are also desired. The usual source of electrons is a thermionic emission cathode, which is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000° C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. The necessity of heating thermionic cathodes to such high temperatures creates several problems. For example, the heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and tends to interfere with high-speed modulation of emission in gridded tubes.
An attractive alternative is field emission at room temperature, which is possible using suitable cold cathode materials. Conventional cold cathode materials are typically made of Spindt-type cathodes formed from either metal (such as Mo) or semiconductor (such as Si), with sharp tips in nanometer sizes. (See I. Brodie and C. Spindt, supra.) Unfortunately, while useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions, and this high voltage operation both increases the damage incurred by the emitter tips and also requires a supply of significant power densities. In addition, fabrication is complicated and costly, particularly for uniform tips across a large area. Thus, vacuum microelectronic devices that incorporate Spindt cathodes tend to suffer from various drawbacks. As an alternative cold cathode material, carbon nanotubes have recently emerged as a potentially useful emitter material. Nanotubes' high aspect ratio (>1,000) and small tip radii of curvature (˜10 nm), coupled with their high mechanical strength and chemical stability, make them particularly attractive as electron field emitters.
For these reasons, improved vacuum microelectronic device designs that avoid current problems are desired, particularly designs incorporating improved cold cathode electron emitters.